Turbofan engine with variable bypass nozzle exit area and method of operation

ABSTRACT

A turbofan engine includes core and fan nacelles that provide a bypass flow path having a nozzle exit area. The bypass flow path carries a bypass flow to be expelled from the nozzle exit area. A turbofan is arranged within the fan nacelle and upstream from the core nacelle for generating the bypass flow. A flow control device includes a surface in the bypass flow path including an aperture. The flow device is adapted to introduce a fluid into the bypass flow path for altering a boundary layer of the bypass flow that effectively changes the nozzle exit area. In one example, bleed air is introduced through the aperture. In another example, pulses of fluid from a Helmholz resonator flow through the aperture. By decreasing the boundary layer, the nozzle exit area is effectively increased. By increasing the boundary layer, the nozzle exit area is effectively decreased.

This application claims priority to PCT Application Serial No.PCT/US2006/039993, filed on Oct. 12, 2006.

BACKGROUND OF THE INVENTION

This invention relates to a turbofan engine, and more particularly, theinvention relates to effectively changing a nozzle exit area of a bypassflow path.

A typical turbofan engine includes a spool supporting a compressor and aturbine. The spool, compressor and turbine are housed within a corenacelle. A turbofan, or “fan,” is coupled to the spool and is arrangedupstream from the core nacelle. A fan nacelle surrounds the turbofan andcore nacelle. The fan and core nacelles provide a bypass flow pathhaving a nozzle exit area through which bypass flow from the fan exitsthe engine.

Turbofan engines typical have a fixed nozzle exit area. The flow throughthe nozzle affects, for example, the operational line of the fan andcompressor and the overall performance and efficiency of the engine.Since the nozzle exit area is fixed, the operational lines and otherengine operating characteristics must be managed using a more limitednumber of engine parameters. The engine parameters are varied duringengine operation to obtain desired engine operating characteristics,such as fuel efficiency. What is needed is a method and apparatus ofmanaging engine operating characteristics by using the nozzle exit areaas an additional variable parameter. What is also needs is an ability touse the nozzle exit area as a variable parameter with minimal cost andweight penalties.

SUMMARY OF THE INVENTION

A turbofan engine includes core and fan nacelles that provide a bypassflow path having a nozzle exit area. In one example, the nozzle exitarea is fixed providing a physically constant size. The bypass flow pathcarries a bypass flow circumventing the core nacelle and expelled fromthe nozzle exit area. A turbofan is arranged within the fan nacelle andupstream from the core nacelle for generating the bypass flow. A flowcontrol device includes a surface in the bypass flow path including anaperture. The flow device is adapted to introduce a fluid into thebypass flow path for altering a boundary layer of the bypass flow thateffectively changes the nozzle exit area. In one example, bleed air isintroduced through the aperture. In another example, pulses of fluidfrom a Helmholz resonator flow through the aperture. By decreasing theboundary layer, the nozzle exit area is effectively increased. Byincreasing the boundary layer, the nozzle exit area is effectivelydecreased.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an example turbofan engine.

FIG. 2 a is a schematic partial side cross-sectional view of a turbofanengine with an example flow control device expelling fluid in a firstmanner.

FIG. 2 b is a schematic partial end view of the turbofan engine shown inFIG. 2 a.

FIG. 3 a is a schematic partial side cross-sectional view of a turbofanengine with the example flow control device expelling fluid in a secondmanner.

FIG. 3 b is a schematic partial end view of the turbofan engine shown inFIG. 3 a.

FIG. 4 is a schematic partial side cross-sectional view of the turbofanengine with another example flow control device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A geared turbofan engine 10 is shown in FIG. 1. A pylon 38 secures theengine 10 to an aircraft. The engine 10 includes a core nacelle 12 thathouses a low spool 14 and high spool 24 rotatable about an axis A. Thelow spool 14 supports a low pressure compressor 16 and low pressureturbine 18. In the example, the low spool 14 drives a turbofan 20through a gear train 22. The high spool 24 supports a high pressurecompressor 26 and high pressure turbine 28. A combustor 30 is arrangedbetween the high pressure compressor 26 and high pressure turbine 28.Compressed air from compressors 16, 26 mixes with fuel from thecombustor 30 and is expanded in turbines 18, 28.

In the examples shown, the engine 10:1 is a high bypass turbofanarrangement. In one example, the bypass ratio is greater than 10, andthe turbofan diameter is substantially larger than the diameter of thelow pressure compressor 16. The low pressure turbine 18 has a pressureratio that is greater than 5:1, in one example. The gear train 22 is anepicycle gear train, for example, a star gear train, providing a gearreduction ratio of greater than 2.5:1. It should be understood, however,that the above parameters are only exemplary of a contemplated gearedturbofan engine. That is, the invention is applicable to other enginesincluding direct drive turbofans.

Airflow enters a fan nacelle 34, which surrounds the core nacelle 12 andturbofan 20. The turbofan 20 directs air into the core nacelle 12, whichis used to drive the turbines 18, 28, as is known in the art. Turbineexhaust E exits the core nacelle 12 once it has been expanded in theturbines 18, 28, in a passage provided between the core nacelle and atail cone 32.

The core nacelle 12 is supported within the fan nacelle 34 by structure36, which are commonly referred to as upper and lower bifurcations. Agenerally annular bypass flow path 39 is arranged between the core andfan nacelles 12, 34. The example illustrated in FIG. 1 depicts a highbypass flow arrangement in which approximately eighty percent of theairflow entering the fan nacelle 34 bypasses the core nacelle 12. Thebypass flow B within the bypass flow path 39 exits the fan nacelle 34through a nozzle exit area 40.

For the engine 10 shown in FIG. 1, a significant amount of thrust may beprovided by the bypass flow B due to the high bypass ratio. Thrust is afunction of density, velocity and area. One or more of these parameterscan be manipulated to vary the amount and direction of thrust providedby the bypass flow B. In one example, the engine 10 includes a structureassociated with the nozzle exit area 40 to change the physical area andgeometry to manipulate the thrust provided by the bypass flow B.However, it should be understood that the nozzle exit area may beeffectively altered by other than structural changes, for example, byaltering the boundary layer, which changes the flow velocity.Furthermore, it should be understood that any device used to effectivelychange the nozzle exit area is not limited to physical locations nearthe exit of the fan nacelle 34, but rather, includes altering the bypassflow B at any suitable location.

The engine 10 has a flow control device 41 that is used to effectivelychange the nozzle exit area. In one example, the flow control device 41provides the fan nozzle exit area 40 for discharging axially the bypassflow B pressurized by the upstream turbofan 20 of the engine 10. Asignificant amount of thrust is provided by the bypass flow B due to thehigh bypass ratio. The turbofan 20 of the engine 10 is designed for aparticular flight condition, typically cruise at 0.8 M and 35,000 feet.The turbofan 20 is designed at a particular fixed stagger angle for anefficient cruise condition. The flow control device 41 is operated tovary the nozzle exit area 40 to adjust fan bypass air flow such that theangle of attack or incidence on the fan blade is maintained close todesign incidence at other flight conditions, such as landing andtakeoff. This enables desired engine operation over a range of flightcondition with respect to performance and other operational parameterssuch as noise levels. In one example, the flow control device 41 definesa nominal converged position for the nozzle exit area 40 at cruise andclimb conditions, and radially opens relative thereto to define adiverged position for other flight conditions. The flow control device41 provides an approximately 20% change in the exit nozzle area 40.

Referring to FIGS. 2 a-4, a flow control device is shown that uses afluid, such as air, to vary a boundary layer Q within the bypass flowpath 39 to effectively change the nozzle exit area 40. The boundarylayer Q is created by the bypass flow B along the walls of the bypassflow path 39.

In the examples shown in FIGS. 2 a-3 b, the flow control device 41 usesbleed air L from one of the compressor stages 54. The bleed air L isintroduced to the bypass flow path 39 in a desired manner to affect theboundary layer Q. It is typically desirable to extract bleed air L fromthe lowest usable compressor stage to minimize the efficiency impact onthe engine. In one example, the compressor stage 54 corresponds to anupstream compressor stage on the high compressor 26. In one example,extraction of bleed air L is avoided during particular engine operatingconditions, such as cruise.

In one example, a controller 50 commands a valve 55 arranged in apassage 52. The passage 52 fluidly connects the compressor stage 54 toapertures 56 arranged on a surface 57 adjacent to the bypass flow path39. Three apertures 56 are shown for exemplary purposes. The apertures56 can be arranged in an array and plumbed in any suitable manner. Thevalve 55 selectively regulates the bleed air L provided through theapertures 56 in response to commands from the controller 50 to obtain adesired boundary layer thickness. The controller 50 determines whenchanges in the effective nozzle exit area 40 are desired for aparticular engine operating characteristic.

Decreasing the boundary layer at the surface 57 effectively “opens” thenozzle exit area 40. A decrease in boundary layer Q increases the meanvelocity of bypass flow B across the nozzle exit area 40. Conversely,decreasing the boundary layer Q at the surface 57 effectively “closes”the nozzle exit area 40. An increase in boundary layer decreases themean velocity of bypass flow B across the nozzle exit area 40.

In the example shown in FIGS. 2 a-2 b, the apertures 56 introduce thebleed air L in a direction generally perpendicular to the bypass flow B,which effectively increases the boundary layer Q and provides andeffective closing of the nozzle exit area 40. The bypass flow B in FIGS.2 b and 3 b are indicated in a generally axial direction.

In the example shown in FIGS. 3 a-3 b, the apertures 56′ are arrangedgenerally tangentially to the bypass flow B so that introducing thebleed air L effectively opens the nozzle exit area 40 by decreasing theboundary layer.

In either approach shown in FIGS. 2 a-2 b and FIGS. 3 a-3 b, the bleedair L provides a range of effective nozzle exit areas 40 betweenno-bleed flow and bleed flow conditions. Said another way, in oneexample, one of the aperture orientations shown in the Figures ischosen. With the chosen aperture configuration, the flow of bleed air Lis adjusted to obtain the desired boundary layer Q. In this manner, theflow control device 41 provides another engine parameter by which theengine operating characteristics can be managed.

Another example flow control device 41′ is shown in FIG. 4. In oneexample, the flow control device 41′ uses a chamber 58 to provide pulsedflow to the surface 57 through passages 60 to the apertures 56. In oneexample, the chamber 58 is tuned to provide air to the bypass flow path39 at a desired frequency. An exciter 64 is actuated by a driver 66 inresponse to a command from the controller 50. The exciter 64 createpulses that are delivered to through the apertures 56 to change theboundary layer Q. The driver 66 modulates the exciter 64 at a desiredfrequency to obtain a desired boundary layer Q. The aperture 56 andchamber 58 geometry are selected to achieve the desired boundary layerQ. The apertures 56 can be arranged in any suitable manner, for examplein the manner described above relative to FIGS. 2 a-3 b.

Although an example embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

The invention claimed is:
 1. A turbofan engine comprising: core and fannacelles providing a bypass flow path having a nozzle exit area, thebypass flow path for carrying a bypass flow to be expelled from thenozzle exit area; a turbofan arranged within the fan nacelle andupstream from the core nacelle for generating the bypass flow; a spoolhaving a turbine mounted thereon and housed within the core nacelle; agear train interconnecting the turbofan and the spool, the turbofancoupled to the spool through the gear train; a flow control deviceincluding a surface in the bypass flow path including an aperture, theflow device adapted to introduce a fluid into the bypass flow paththrough the aperture for altering a boundary layer of the bypass flowthat effectively changes the nozzle exit area; and a controller incommunication with the flow control device, the controller configured todetermine when chan es in the effective nozzle exit area are desired andcommand the flow control device to obtain a desired boundary layer. 2.The turbofan engine according to claim 1, comprising a flow source forproviding the fluid.
 3. The turbofan engine according to claim 2,comprising a compressor arranged within the core nacelle, the compressorproviding bleed air as the fluid.
 4. The turbofan engine according toclaim 3, comprising a low spool and a high spool rotatable relative toone another and housed within the core nacelle, the compressor mountedon the high spool, and the turbofan coupled to the low spool through thegear train.
 5. The turbofan engine according to claim 1, wherein theflow control device includes a controller programmed to command a valvefor regulating a flow of the fluid through the aperture, wherein thevalve is closed during a cruise condition.
 6. The turbofan engineaccording to claim 1, wherein the aperture is arranged to introduce thefluid generally perpendicularly to the bypass flow for increasing theboundary layer.
 7. The turbofan engine according to claim 1, wherein thebypass flow path extends axially along a radial space arranged betweenthe core and fan nacelles.
 8. A turbofan engine comprising: core and fannacelles providing a bypass flow path having a nozzle exit area, thebypass flow path for carrying a bypass flow to be expelled from thenozzle exit area; a turbofan arranged within the fan nacelle andupstream from the core nacelle for generating the bypass flow; a flowcontrol device including a surface in the bypass flow path including anaperture, the flow device adapted to introduce a fluid into the bypassflow path through the aperture for altering a boundary layer of thebypass flow that effectively changes the nozzle exit area, wherein theaperture is arranged to introduce the fluid generally in the samedirection as the bypass flow for decreasing the boundary layer; acontroller in communication with the flow control device, the controllerconfigured to determine when changes in the effective nozzle exit areaare desired and command the flow control device to obtain a desiredboundary layer; and a compressor arranged within the core nacelle, thecompressor providing bleed air as the fluid.
 9. The turbofan engineaccording to claim 8, wherein the bypass flow path extends axially alonga radial space arranged between the core and fan nacelles.
 10. A methodof controlling a turbofan engine comprising the steps of: determiningwhen changes in the effective nozzle exit area of a turbofan bypass flowpath are desired; commanding a flow control device to obtain a desiredboundary layer by introducing a compressor bleed air into a turbofanbypass flow path to alter a bypass flow through the bypass flow path;and effectively changing a nozzle exit area of the bypass flow path withthe altered bypass flow, including decreasing a boundary layer along asurface within the bypass flow path thereby obtaining the desiredboundary layer.
 11. The method according to claim 10, wherein the stepof effectively changing the nozzle exit area includes increasing aboundary layer along a surface within the bypass flow path.
 12. Themethod according to claim 10, wherein the bypass flow path extendsaxially along a radial space arranged between a core nacelle and a fannacelle.